Both are excellent cartridges for deer hunting,i've used both and seen both take a lot of deer.

My thoughts on why the 7mm-08 Rem is more popular than the 280 Rem. which has better ballistics.

Because of the popularity of the short action with less recoil in the silhouette shooting sport which then spilled into the hunting community searching for a efficient cartridge w/less recoil has made the 7mm-08 Remington much more popular than the 280 Remington for deer hunting.

My Dad has had a Rem 721 in a 280 Rem. since the late 50s early 60s? and its been a deer slayer,I love that old rifle!

Fourteen years ago it was time to set my son up for deer hunting,I chose the 7mm-08 Rem.
My decision and reasons for the 7mm-08 Rem for a nine year old
1) Less recoil,I also handload so the first practice batches and first year of hunting ammo were reduced loads which helped a lot for a young shooter.
2) The short action rifles are a bit smaller for younger and smaller hunters
3) I'm also a 7mm(.284) fan when it comes to deer/elk hunting cartridges.